1. Field of the Invention
The present invention relates to a circularly polarized antenna and, more particularly, to a small-sized circularly polarized antenna for transmitting and receiving a circularly polarized signal.
2. Description of Related Art
In some electrical devices, such as the reader device of an RFID system, the antenna module of which must be able to transmit and receive a circularly polarized signal, in order to ensure that the electrical devices can operate normally in any kind of attitude. Besides, since the antenna module must be small enough to be portable, the size of the antenna module of the electrical device is also limited.
Generally, the circularly polarized antenna of the prior art uses a straight coupling line to couple the electrical signal to the antenna unit, in order to transform the electrical signal into a circularly polarized signal. Then, the circularly polarized signal is transmitted outside. Thus, the substrate of the circularly polarized antenna of the prior art must have a size large enough to enclose the straight coupling line on the surface thereof. Moreover, since the length of the side of the antenna unit must be about half of the wavelength of the circularly polarized signal being transmitted, so if the frequency of the circularly polarized signal being transmitted is 915 MHz, the length of the antenna unit should be 164 mm in the free space.
The methods to reduce the length of the side of the antenna unit are (1) forming some slots on the surface of the antenna unit or (2) changing the shape of the antenna unit, in order to increase current path. But, both the aforementioned methods are too complex. As a result, the structure of the circularly polarized antenna of the prior art is too complex to reach the requirement of easy-design.
Therefore, it is desirable for the industries to provide a circularly polarized antenna with a small size, which can not only have the simple structure (the standard shape of square and circle), but also have same function to apply in any kind of the antenna module of the portable electrical device.